1. Field of the Invention
The present invention relates generally to printing objects that can be stacked, for example compact disks and digital versatile disks.
2. Description of the Prior Art
The present invention addresses the situation in which a printing machine used to print such objects includes a circular contour printing table with a plurality of object-stations distributed around its periphery each adapted to receive an object to be printed. Mounted to rotate stepwise about a vertical axis, it moves the object-stations successively to a loading station, to a plurality of workstations each including printing means, and to an offloading station. A loading system at the loading station includes a presentation system adapted to feed at least one stack of objects to the loading station and a transfer system operative between the presentation system and the printing table and equipped with at least one pick-up unit adapted to pick up an object from the stack on the presentation system and place that object on one of the object-stations of the printing table.
In practice there is usually one workstation for each color to be printed and, at the loading station, the transfer system of the loading system takes up the objects on the presentation system one by one and puts them down on the printing table one by one.
The overall rate of production is therefore determined by the rate of operation of the loading system.
The document FR-A-2 714 867 describes a printing machine which is of the kind described above and in which the loading system at the loading station includes two presentation systems for the same transfer system; the transfer system includes at least one pair of pick-up units disposed side-by-side one of which co-operates with one presentation system and the other of which cooperates with the other presentation system, in succession.
The transfer system therefore picks up an object to be printed from each presentation system in succession and then puts down the two objects to be printed that it has picked up in this way on the printing table simultaneously.
Assuming that the number of workstations is an even number 2N, it is therefore advantageously possible to print N colors on two objects at a time during one and the same rotation of the printing table, half of the workstations successively printing the same object while the other half of the workstations are successively printing another object at the same time.
The overall rate of production is therefore doubled, which is to the benefit of productivity.
Alternatively, on this kind of printing machine, assuming that the number of object-stations on the printing table is odd and that the working area of each workstation corresponds to an even number of object-stations, it is advantageously possible to print 2N colors successively on one object at a time using the 2N workstations and in the course of two successive rotations of the printing table.
The production rate is halved compared to the previous rate, but the printing machine then has the advantage of being more flexibly adaptable to different printing conditions, for example the number of objects to be printed during the same run and/or the number of colors to be printed on the objects.
In practice, an offloading system is used at the offloading station and is of similar design to the loading system; the offloading system includes a transfer system and, associated therewith to provide the possibility of doubling up production, two evacuation systems which are of the same type as the presentation systems of the loading system and each of which is adapted to accumulate at least one stack of objects and to move it away from the offloading station.
In a process that is similar to that previously described for the loading station, the transfer system simultaneously picks up from the printing table two objects that have already been printed and normally releases one of them at one evacuation system and the other at the other evacuation system.
An object of the present invention is to enable a machine of the above type to print both sides of an object such as a digital versatile disk.
A printing machine in accordance with the invention includes a circular contour printing table which has a plurality of object-stations regularly distributed around its periphery each adapted to receive an object to be printed and which, rotatable stepwise about an axis, moves the object-stations successively to a loading station, to a plurality of workstations each including a printing system, and to an offloading station, wherein it is associated with a system for turning over the objects which includes a first transfer arm adapted to pick up an object on the printing table and put it down on an overturning shovel which is adapted to turn it over and to put it down at a receiving station and a second transfer arm adapted to pick up the turned-over object at the receiving station and to put it back down on the printing table.
The transfer arms are advantageously separated by a distance at least equal to the pitch of the object-stations.
Each transfer arm is preferably carried by a carriage sliding on a rail and the transfer arm slides vertically on the carriage.
Each carriage is advantageously coupled to a link rotatable about an axis carried by the carriage and whose end opposite that by which the carriage is coupled to it is articulated to a crank driven by an electric motor.
The axis of the link is preferably offset relative to the plane in which the arm slides vertically.
The transfer arm is advantageously caused to slide vertically by a link coupled at one end to the arms and at the other end to the end of the link.
The two cranks are preferably opposed and parallel so that when the first transfer arm is closest to the periphery of the printing table 11 the second arm is farthest away from it.
The two cranks are advantageously driven by a single electric motor.
The overturning shovel is preferably rotatable about the axis of a gear motor adapted to cause the overturning shovel to assume two positions spaced by 180xc2x0, that is to say a receiving position and a feed position.
The free end of the overturning shovel advantageously carries suction nozzles.
The electric motors are preferably digitally controlled brushless motors.
The printing machine preferably includes a loading system at the loading station which includes a presentation system adapted to feed at least one stack of objects to the loading station and a transfer system operative between the presentation system and the printing table and equipped with at least one pick-up unit adapted to pick up an object on the stack present on the presentation system and then to put that object down on one of the object-stations of the printing table.
The transfer system advantageously has three pickup units spaced by 120xc2x0.
The presentation system preferably includes a circular contour plate rotatable stepwise about an axis parallel to the rotation axis of the printing table and having a plurality of circumferentially distributed object-stations each adapted to receive a stack support.
The printing machine advantageously includes an offloading system at the offloading station of similar construction to the loading system, the offloading system including a transfer system and associated therewith an evacuation system adapted to accumulate at least one stack of objects and to move it away from the offloading station.
The printing machine preferably includes an even number of workstations.
The printing table advantageously has an odd number of object-stations and the working area of each workstation corresponds to an even number of object-stations.
The features and advantages of the invention will become apparent from the following description, which is given by way of example and with reference to the accompanying diagrammatic drawings.